1. Field of the Invention
The present invention relates to a magnetic encoder for use with slide calipers, a micrometer, and so forth, in particular, to a magnetic encoder for detecting a variation of magnetic coupling of members that relatively move so as to detect the relative displacement (positions) thereof.
2. Prior Art
As an example of an encoder that detects the position, angle, and so forth of members that relatively moves, a magnetic encoder is known. A magnetic encoder comprises a first member and a second member. The first member has N pole portions and S pole portions that are arranged alternately at predetermined pitches .lambda.. The first member is for example a magnetic scale. The second member is disposed so as to be relatively movable to the first member. The second member has devices that detect the variation of the magnetic field corresponding to the relative movement to the members. For example, first to fourth magnetoresistance (MR) devices with phases that differ from by .lambda./4 (=90.degree.) each are disposed corresponding to the pitches (magnetizing pitches) .lambda. of the magnetic scale.
The first to fourth MR devices on the second member are connected in such a manner that MR devices with phases that differ from by 180.degree. are connected in series. With the first to fourth MR devices connected in such a manner, a displacement detecting circuit is structured as a bridge circuit. The first to fourth MR devices are magnetized in the longitudinal direction perpendicular to the direction of the movement of the member. With the interaction of DC currents supplied to these MR devices and the horizontal magnetic field of the magnetic scale, the magnetization is rotated, thereby causing the resistance of these MR devices to vary. The intensity of the horizontal magnetic field applied to each MR device periodically varies at the pitches .lambda. corresponding to the relative movement. Thus, the rotation angle of the magnetization periodically varies. Consequently, sine wave signals with phases that differ from by 90.degree. are obtained from output terminals of the bridge circuit. By processing the sine wave signals, the amount of displacement can be obtained.
In a magnetic encoder having such MR devices, the influence of water, cutting oil, and so forth to output signals is smaller than that in an electrostatic type encoder and a photoelectric type encoder. Thus, the magnetic encoder is especially suitable for applications that require environmental resistance as in a machining plant.
When the magnetic encoder is used for a battery-driven hand-tool length measuring unit or the like, the power consumption is a critical problem. In other words, a DC power supply is connected to the MR devices that compose the displacement detecting circuit. Thus, a drive current is supplied from the DC power supply to the MR devices. For example, when the resistance of each MR device is 1.5 k .OMEGA. and the applied voltage is 1.5 V, since the total resistance of the bridge circuit is 1.5 k .OMEGA., the current that flows in the bridge circuit amounts to 1 mA. Thus, for example, the service life of a silver-oxide button type battery of 160 mAh is as small as 160 hours (around 6.5 days).
In such a magnetic encoder, as a substrate on which MR devices are formed, a glass substrate or a ceramic substrate is used. On the other hand, when a signal processing circuit that processes output signals of the MR devices is structured as an IC chip, to dispose the integrated circuit and the MR devices, there are several methods such as methods (a) and (b). In the method (a), a substrate on which MR devices are formed and a substrate on which an integrated circuit is formed are connected with a flexible print circuit (FPC) board. In the method (b), an IC chip is disposed on the rear surface of a substrate on which MR devices are patterned.
However, in the conventional mounting methods, since the mounting size is large, it is difficult to accomplish a small hand-tool. Although the mounting density in the method (b) where one substrate is used in common is larger than the mounting density in the method (a) where two substrates on which MR devices and an integrated circuit are formed, since the MR devices and the integrated circuit use different areas, the reduction of the size is restricted. In addition, the surface on which the MR devices are formed should be disposed as a reference surface opposite to the magnetic scale with a predetermined gap. However, when the integrated circuit is disposed, the reference surface cannot be precisely obtained. When the MR devices are disposed opposite to the magnetic scale, the protecting structure against contamination becomes complicated.
The conventional magnetic encoder having MR devices with different phases are largely affected by fluctuation of the accuracy of the magnetizing pitches and intensity of magnetic field, fluctuation of the positions, shapes, and characteristics of the MR devices, and so forth. Thus, on the electric signal processing circuit side, the center voltage, amplitude, and so forth of the sine wave signals should be adjusted. In addition, since the magnetic encoder is largely affected by a damage and contamination of MR device pattern, the stability and reliability thereof are not high. Moreover, as the magnetizing pitches of the scale is becoming small, it is difficult to dispose four MR devices in the magnetizing pitches. Thus, when the MR devices are precisely machined, the fluctuation of characteristics thereof becomes large.